A polycarbonate resin has excellent characteristics such as strength, impact resistance, transparency, etc. and is widely used as an engineering resin in the production of, e.g., electrical parts, mechanical parts, and containers.
The usual linear polycarbonate resin, however, has the features that, when melted, it behaves nearly as a Newtonian liquid and its apparent viscosity does not depend on a rate of shear, and furthermore, its melt elasticity and melt strength are poor. For this reason, in extrusion molding of polycarbonate, particularly blow molding using an extrusion molding machine, it is difficult to produce a stable and large-sized parison.
As methods of improving the above-described disadvantages of polycarbonate, a method of mixing two types of polycarbonate having significantly different molecular weights and a method of branching polycarbonate are known. The latter branched polycarbonate-producing method includes (1) a method of blowing phosgene into dihydric phenols and polyfunctional compounds (see Japanese Patent Publication No. 17149/1969, and (2) a method of effecting the procedure of the method (1) above in the presence of monophenols (see Japanese Patent Publication No. 23918/1972). These methods, however, are not preferred because the polyfunctional compounds are of low reactivity. Thus, there have been proposed (3) a method in which polycarbonate oligomers having a degree of polymerization of from 5 to 15 are prepared and, thereafter, reacted by the addition of a catalyst and polyphenol (see Japanese Patent Publication No. 28193/1978), and (4) a method in which the first-half step of blowing phosgene in the method (2) as described above is performed under the condition that the pH is adjusted to from 3 to 6 (see Japanese Patent Application Laid-Open No. 129493/1976). These methods, however have, various disadvantages; in the method (3), it is difficult to control the process so that the reaction of the added polyphenol is completed, and in the method (4), the dihydric phenol easily precipitates (or deposits) and, therefore, the equipment that can be used is limited and a tubular reactor cannot be used.